The present invention relates to an inverter adapted to controllably drive an AC motor and more particularly to a method and an apparatus for control of the inverter which are directed to minimization control of motor current.
As an extremum search circuit used for a process control system and the like, one shown in JP-A-58-191004 is available. The disclosed circuit searches for an extremum of a controlled variable in a control system in which the extremum of the controlled variable varying with a manipulated variable changes in accordance with a disturbance, and it is based on such a principle that a difference between a current controlled variable (value) and a preceding controlled variable (value) is determined every cycle at a predetermined period, the manipulated variable is further changed in the same direction as that in the preceding cycle if the difference value, i.e., the change occurs in a direction for approach to an extremum, but the manipulated variable is changed in a direction opposite to that in the preceding cycle if the change occurs in a direction for departure from the extremum, and the above sequential operation is repeated to cause the controlled variable to be controlled so as to approach the extremum.
On the other hand, a method for minimization control of motor current is known which is based on a similar principle to the above, whereby output voltage of an inverter is changed at a sampling time point, motor current is detected when it subsequently becomes stable, a difference between a detected value at a preceding sampling time point and a current detected value is determined, and the output voltage of the inverter is changed in the same direction as that in the preceding cycle if the motor current changes in its decreasing direction, but the output voltage of the inverter is changed in a direction opposite to that in the preceding cycle if the motor current changes in its increasing direction, thus causing the motor current to be controlled so as to approach a minimum value. As an example of the method for minimization control of motor current, a prior art method as shown in JP-A-62-51781 will now be described.
FIG. 28 shows a block diagram of the prior art inverter apparatus. In the figure, reference numeral 1 designates a commercial AC power source, 2 a converter for converting the output of the AC power source 1 into direct current, 3 a DC smoothing circuit for smoothing the DC output of the converter 2, 4 an inverter for converting direct current from the DC smoothing circuit 3 into alternating current, 5 an AC motor driven by the inverter 4, 7 a winding current detector/amplifier for detecting and amplifying input current to the winding of the AC motor 5, 11 a DC variable converter for converting a detected motor current value into a DC variable to detect an average value of the motor current, 8' a memory for storing the DC variable from the DC variable converter, 9' a comparator for comparing data representative of the DC variable from the DC variable converter 11 with stored current value data, 12' a pattern shift operation unit adapted to deliver a shift instruction for shifting an output voltage pattern on the basis of output data from the comparator 9' and a frequency command f* from a frequency command unit 14', and 16' an output voltage pattern storage unit in which a number of output voltage patterns of different output voltages V having a constant ratio between their magnitudes and frequencies F are stored and which responds to the frequency command f* and the shift instruction to deliver an output voltage command. Denoted by 9 is a PWM signal generator for generating a pulse width modulated signal (PWM signal) on the basis of the output voltage command, and by 10 is a drive circuit for driving switching elements of the inverter 4 on the basis of the PWM signal.
In the conventional current minimization control, running proceeds under a given output voltage pattern A and a motor current value at that time is detected and stored. Subsequently, running proceeds under an output pattern (A+1) and after a predetermined time delay, a motor current value at that time is compared with the previously stored motor current value If a decrease in the current is determined, the output voltage pattern is shifted to (A+2) of increased voltage. Conversely, if an increase in the current value is determined when the output voltage pattern is shifted from A to (A+1), the output voltage pattern is shifted to (A-1) of decreased voltage. In this manner, the output voltage pattern is shifted in a direction for decreasing the current value and when the current value increases from the preceding current value, the preceding output voltage pattern is decided to be optimum and is then fixed. This value is kept until the output frequency changes.
The conventional extremum search method and inverter control apparatus for minimization of motor current have constructions described as above and therefore they face a problem that when the disturbance or motor load changes or when the motor runs with its speed increased or decreased, an optimum manipulated variable (value) at which an extremum of the controlled variable is given or an optimum output voltage at which the motor current is minimized cannot be searched out. In the case of an inverter apparatus for minimization of, for example, motor current as the load decreases continuously and the motor current value decreases sympathetically, the motor current decreases regardless of the direction in which the output voltage of the inverter changes. Since in this case it is impossible to decide whether the motor current decreases on account of a decrease in load or on account of a change in output voltage, there sometimes occurs an inconvenience that the output voltage is fixed to a value indicative of not-optimum voltage or a divergent phenomenon that the output voltage is caused to continue increasing or decreasing Similar problems also take place when the motor current value is erroneously detected owing to noise and the like.